In general, an organic electroluminescent device (hereinafter referred to as organic EL device) is constructed of a light-emitting layer and a pair of counter electrodes interposing the light-emitting layer therebetween in its simplest structure. That is, the organic EL device uses the phenomenon that, when an electric field is applied between both the electrodes, electrons and holes are injected from a cathode and an anode, respectively, and each electron and each hole recombine in the light-emitting layer to emit light.
In recent years, progress has been made in developing an organic EL device using an organic thin film. In particular, development has been made in order to enhance luminous efficiency. In the development, the efficiency of injection of carriers from electrodes has been improved through optimization of the kind of the electrodes. In addition, there has been developed a device using a hole-transporting layer formed of an aromatic diamine and a light-emitting layer also serving as an electron-transporting layer formed of an 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3), resulting in a significant improvement in luminous efficiency, as compared to related-art devices. Thus, the development of the organic EL device has been promoted in order to accomplish its practical application to a high-performance flat panel having features such as self-luminescence and rapid response.
Investigations have been made on using a phosphorescent light-emitting material rather than a fluorescent light-emitting material as an attempt to improve the luminous efficiency of a device. Many kinds of devices including the device in which a hole-transporting layer formed of an aromatic diamine and a light-emitting layer formed of Alq3 are formed emit light by using fluorescent light emission. However, by using phosphorescent light emission, that is, light emission from a triplet excited state, luminous efficiency is expected to be improved by from about three times to four times, as compared to the case of using the related-art devices in which fluorescent light emission (light emission from a singlet excited state) is used. In order to accomplish this purpose, investigations have been made on using a coumarin derivative or a benzophenone derivative in a light-emitting layer, but extremely low luminance has only been provided. Then, investigations have been made on using a europium complex as an attempt to use a triplet excited state, but highly efficient light emission has not been accomplished. As the investigations using phosphorescent light emission, many investigations have been made on using an iridium complex or the like as disclosed in Patent Literature 1 as a phosphorescent light-emitting dopant, and some materials having high luminous efficiency have been found.